# std::is_standard_layout

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Defined in header `<type_traits>` |  |
template< class T > struct is_standard_layout; |  | (since C++11)

`std::is_standard_layout` is a UnaryTypeTrait.

If `T` is a standard-layout type, provides the member constant `value` equal to true. For any other type, `value` is false.

If std::remove_all_extents_t<T> is an incomplete type and not (possibly cv-qualified) void, the behavior is undefined.

If the program adds specializations for `std::is_standard_layout` or `std::is_standard_layout_v`, the behavior is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > constexpr bool is_standard_layout_v = is_standard_layout<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is a standard-layout type, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Notes

A pointer to a standard-layout class may be converted (with reinterpret_cast) to a pointer to its first non-static data member and vice versa.

If a standard-layout union holds two or more standard-layout structs, it is permitted to inspect the common initial part of them.

The macro offsetof is only guaranteed to be usable with standard-layout classes.

### Example

Run this code

    #include <type_traits>

    struct A { int m; };
    static_assert(std::is_standard_layout_v<A> == true);

    class B: public A { int m; };
    static_assert(std::is_standard_layout_v<B> == false);

    struct C { virtual void foo(); };
    static_assert(std::is_standard_layout_v<C> == false);

    int main() {}

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2015 | C++11 | `T` could be an array of incomplete class type with unknown bound | the behavior is undefined in this case

### See also

is_trivially_copyable (C++11) | checks if a type is trivially copyable (class template)
is_pod (C++11)(deprecated in C++20) | checks if a type is a plain-old data (POD) type (class template)
offsetof | byte offset from the beginning of a standard-layout type to specified member (function macro)

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